Method of cleaning coal fired boiler installations



May 20, 1941. K. L. HAGEMANS 2,242,354 METHOD OF CLEANING COAL FIRED BOIL ER INSTALLATIONS Filed Aug. 29 1939 mamt fz f boiler installations.

Patented May 20; 194i METHOD OF CLEANING COALFIRED'BOILER INSTALLATIONS erlands Application August 29, 1939, Serial N0.'292,539 I In Germany February-16, 1938 6 Glaims.

In coal fired boiler installations it is necessary from time to time to remove flue dust deposited on the surfaces in direct contact with the combustion gases, for instance thesurfaces of Water tubes, economizers, air heaters etc., as this dust reduces the heat transmission and increases the resistance to flow of said gases. If, as is ordinary practice, steam jets are used for this purpose, steam may condense on relatively cold surfaces, for instance, those of the economizer, whereby said surfaces are liable to get crusted. Cleaning by means of air jets requires a comparatively large compressor and a considerable amount of energy. The admixture of sand with the air still further increases the costs, apart from the fact that sand has an erosive efiect on the metal parts of the installation and is not always easy to procure in the desired quality.

My present invention has for its object a very simple and cheap method of cleaning coal fired It is based upon the discovery that the deposit mainly consists of very fine flue dust, and that the coarser dust particles in the combustion gases have a marked cleaning effect on those parts of the installation with which they come into contact. In accordance with my invention, flue'dust from one or more dust collecting locations of the boiler installation is returned to the path of the combustion gases, preferably at a point Where any combustible present in the flue dust cannot ignite, i. e. where the temperature does not substantially exceed say 1000" C. I

The employment of flue dust, separated from the combustion gases in boiler installation, for

cleaning contaminated parts of said installation has the advantageous feature that use is made of practically worthless material that has already been carried along by the hot gases from the combustion chamber to the dust collecting locations or catchers, i. e. material that is not likely to be deposited on the surfaces of Water tubes or other parts of the installation, or to fall down onto the grate, such as may be the case with sand that has not been thoroughly sieved for size. Besides, the erosive effect of flue dust is not nearly so strong as that of sand.

As a matter of course, the recycled flue dust should be well distributed in the flow of combustion gases. For instance, flue dust from the collecting places may be reintroduced into the boiler at various points by means of a stream of gas, in such a manner as to be evenly distributed in the combustion gases. For this reintroduction, a certain amount of secondary air, orof compractice may be very simple.

pressed 'undergrate "air, or of the combustion gases maybe used. Alternatively, the distribution may be brought about-by means of a separate, small sized lowpressure fan, or the flue dust may simplybe-allowed to be drawn into the boiler by the vacuum obtaining in the installation. The onlycondition that should be'fulfllled is that the flue dust be reintroduced at appropriate points and be caused to evenlydistributeitself in the flow of combustion gases so as to, increase the normalconcentration of flue dust therein. This increased flue dust concentration has a marked cleaning efiect on the contaminated parts of the installation,

The recycling of flue dust may take placeintermittently, or continuously. In the first case, the flue dust-concentration of the combustion gases is In the second case, a comparatively small proportion of the continuously separated flue dust is continuously recycled, said proportion being such as -to just prevent the boiler from being contaminated. Under these conditions, the erosive effect will be minimized. Alternatively, one partof the boiler installation may be cleaned continuouslyand another part intermittently or one part may be subjected to a stronger cleaning action than another.

In installations comprising a plurality of boilers, flue dust from one boiler may be used for cleaning other boilers of the same installation.

As will be'apparent from the above description, the means for carrying my novel method into That is to say, all that is necessary is connecting, directly or indirectly, and through a suitable valved conduit, the outlet of at least one of the dust collecting places with an up-stream point of the path of the combustion gases.

An explanation of how the foregoing method may be carried out is shown by the accompanying drawing wherein:

The figure is a vertical elevation more or less diagrammatically illustrating a coal fired water tube boiler provided with means for carrying the invention into efiect.

Referring specifically to the drawing, I is the mechanical grate, 2 a nest of steam generating water tubes, 3 the superheater, 4 the economizer and 5 the combustion air heater. The hot combustion gases from the furnace 5 flow upwards through the nest of tubes 2 and through the superheater 3, thence downward through the economizer 4 and thence upwards again through an uptake I to the inlet of a main centrifugal dust separator 8. The cleaned gases are discharged from the central portion of said separator and flow downward to the suction side of an induced draft fan 9, by which they are discharged into the chimney Ill. The casing of separator 8 is interrupted by an axial slot II, through which the dust, concentrated in a relatively small, circumferential flow of gas, enters a secondary separator I2. From the latter, the dust falls into a bunker I3 and the cleaned gases are discharged, through a duct l4, into the central portion of the main separator 8.

The fine dust collected in bunker I3 is adapted to be discharged from the bottom thereof by a pipe I5, whereby it is normally returned to the furnace to be fired therein. Said pipe has a branch I6 connected to a plurality of perforated horizontal pipes I1 extending across the path of the combustion gases on their way from the nest of tubes 2 to the superheater 3. A small low pressure blower I8 is adapted to draw in clean combustion gases and to discharge them through a nozzle into pipe I6, the jet of gases thus produced acting as an ejector for drawing dust from bunker I3 and discharging the same at a very low overpressure through the perforations of pipes IT. A valve I5 provided in pipe I5 just below branch I6 serves for shutting off said pipe whenever dust from bunker I3 is'to be discharged through pipes I'I, instead of through pipe I5.

The hot combustion gases flowing from the economizer 4 to the uptake 1 have to pass a settling chamber I9 provided with a hopper for collecting coarse dust entrained by said gases. From said hopper, the dust is adapted to be discharged through a pipe 2|, whereby it is normally returned to the furnace to be fired therein. The outlet opening of hopper 20 is further connected to a pipe 22, which, the same as pipe I6, is connected to the horizontal distributing pipes II. A second low pressure blower 23 adapted to draw in clean combustion gases from the top of settling chamber I9, is arranged to discharge said gases through a nozzle into pipe 22 thereby to eject dust from hopper 20 and discharge it, at a very low overpressure, through the perforations of pipes II. A valve 2| provided in pipe 2| serves for shutting off the same. b

The modus operandi of the described boiler cleaning apparatus is as follows. In normal operation of the boiler, the valves I5 and 2I are open, and the dust (flue coke) entrained by the combustion gases is accumulated in hopper 20 and bunker I3, whence it is discharged through pipes I5 and H to be fired in the furnace. Whenever it is desired for the superheater 3 and the economizer 4 to be cleaned, valves I 5 and Ill are closed for a short time and fans I 8 and 23 driven, whereby a mixture of fine and of coarse dust is discharged, without any appreciable overpressure, through the distributing pipes II into the stream of combustion gases entering the superheater. The dust particles thus discharged very considerably increase the dust concentration of said gases and thereby have a. marked cleaning effect on the contaminated parts of the installation. Vanes 24 serve for keeping the dust particles uniformly distributed throughout the entire cross sectional area of the gaseous flow on its way from the superheater 3 and the economizer 4.

It will be understood that it will not always be necessary for both the fine and the coarse dust to be used for cleaning purposes. It may suffice to only use the fine, or the coarse dust.

What I claim is:

1. In the operation of a coal boiler plant having collecting means positioned in the combustion gas path for collecting flue dust, the method of cleaning undesired dust deposits from the combustion gas path which consists in withdrawing flue dust from said collecting means and reintroducing said withdrawn flue dust into the combustion gas path at a point thereof upstream with reference to the point of said withdrawal where the temperature of the combustion gases is below the ignition temperature of combustible present in the flue dust, whereby the normal concentration of flue dust in the combustion gas is considerably increased.

2. In the operation of a coal boiler plant having collecting means positioned in the combustion gas path for collecting flue dust, the method of cleaning undesired dust deposits from the combustion gas path which consists in withdrawing flue dust from said collecting means and reintroducing said withdrawn flue dust into the combustion gas path at a plurality of spaced points thereof where the temperature of the combustion gases is below the ignition temperature of combustible present in the flue dust, for the even distribution of said flue dust in the combustion gas current.

3. A cleaning method according to claim 1, said steps of withdrawing and reintroducing being performed continuously.

4. A cleaning method according to claim 1, said step of reintroducing being performed intermittently at relatively high concentrations of flue dust in the combustion gas.

5. A cleaning method according to claim 1, said step of reintroducing being performed by applying a stream of gas under low pressure to the withdrawn dust and introducing said stream of gas under low pressure carrying said flue dust into said combustion gas path. a

6. A cleaning method according to claim 1, said step of reintroducing being performed intermittently.

KAREL L. HAGEMANS. 

